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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D12102, doi:10.1029/2008JD011213, 2010
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Summertime precipitation variability over Europe and its
links to atmospheric dynamics and evaporation
Igor I. Zveryaev1 and Richard P. Allan2
Received 29 September 2008; revised 18 November 2009; accepted 12 January 2010; published 16 June 2010.
[1] Gridded monthly precipitation data for 1979–2006 from the Global Precipitation
Climatology Project are used to investigate interannual summer precipitation variability
over Europe and its links to regional atmospheric circulation and evaporation. The first
empirical orthogonal function (EOF) mode of European precipitation, explaining
17.2%–22.8% of its total variance, is stable during the summer season and is associated
with the North Atlantic Oscillation. The spatial‐temporal structure of the second EOF
mode is less stable and shows month‐to‐month variations during the summer season. This
mode is linked to the Scandinavian teleconnection pattern. Analysis of links between
leading EOF modes of regional precipitation and evaporation has revealed a significant
link between precipitation and evaporation from the European land surface, thus,
indicating an important role of the local processes in summertime precipitation variability
over Europe. Weaker, but statistically significant links have been found for evaporation
from the surface of the Mediterranean and Baltic Seas. Finally, in contrast to winter,
no significant links have been revealed between European precipitation and evaporation in
the North Atlantic during the summer season.
Citation: Zveryaev, I. I., and R. P. Allan (2010), Summertime precipitation variability over Europe and its links to atmospheric
dynamics and evaporation, J. Geophys. Res., 115, D12102, doi:10.1029/2008JD011213.
1. Introduction
[2] Variability of precipitation in the European region
on a variety of time scales substantially impacts human
activities. Climate anomalies associated with deficient/
excessive precipitation may lead to serious social and economic consequences. Recently, there were several examples
of such climate anomalies in different parts of Europe
that resulted in significant damage to regional economies
[e.g., Christensen and Christensen, 2003; Schär et al., 2004;
Marsh and Hannaford, 2007; Blackburn et al., 2008,
Lenderink et al., 2009]. Many regional climate extremes
occur during summer. One of the most recent examples of
such extremes is the anomalously high precipitation over
Great Britain during summer 2007, and this resulted in
extensive flooding across England and Wales [Marsh and
Hannaford, 2007; Blackburn et al., 2008]. Nevertheless,
compared to winter, significantly less attention has been
given to analysis of the European climate variability during
the summer season [e.g., Colman and Davey, 1999; Hurrell
and Folland, 2002; Zveryaev, 2004; Zolina et al., 2008]. In
general, summertime climate variability in the European
region is not well studied or understood. Moreover, predictability of the climate in midlatitudes for the summer
1
P.P. Shirshov Institute of Oceanology, RAS, Moscow, Russia.
Department of Meteorology, University of Reading, Reading, UK.
2
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2008JD011213
season shows generally lower skill than that for the winter
season [e.g., Colman and Davey, 1999; Dirmeyer et al.,
2003; Koenigk and Mikolajewicz, 2008]. In particular, on
the basis of analysis of the North Atlantic sea surface temperature anomalies, Colman and Davey [1999] found quite
low skills of statistical predictability of European climate
during summer. Therefore, to improve prediction of regional
climate and its extremes, particularly for the warm season,
further analysis of the processes driving European climate
variability is necessary.
[3] In contrast to winter, when European precipitation
variability is mostly driven by the North Atlantic Oscillation
(NAO) [e.g., Hurrell, 1995; Qian et al., 2000; Zveryaev, 2006],
mechanisms driving interannual variability of regional precipitation during summer are more complex and are not well
understood. In summer, when the role of atmospheric
moisture advection in precipitation variability is diminished,
the role of the local land surface processes increases
[Trenberth, 1999]. Some studies point to the importance of
land surface processes in summer precipitation variability
[Koster and Suarez, 1995; Schär et al., 1999; Seneviratne et
al., 2006], whereas other works highlight the role of the
summer atmospheric circulation [Pal et al., 2004; Koster et
al., 2004; Ogi et al., 2005]. Although the above mechanisms
are not mutually exclusive, there is a high degree of
uncertainty regarding their role in summer precipitation
variability in the Northern Hemisphere extratropics and
particularly over Europe.
[4] The present study focuses on the analysis of the
summer precipitation variability over Europe on an inter-
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annual time scale and on the links between this variability
and regimes of the atmospheric circulation in the Atlantic‐
European sector. While our recent studies [Zveryaev, 2004;
2006] highlighted seasonal differences in precipitation variability over Europe and were based on analysis of seasonal
mean precipitation, the present study examines summer
season evolution of the leading modes of regional precipitation. In other words, we address the question of how stable
are the leading modes of summer season precipitation, a
highly variable (both in time and space) climate parameter.
We also examine stability of the links between the leading
modes of regional precipitation and regimes of atmospheric
circulation during summer season. Note, our recent analysis
[Zveryaev, 2006, 2009] revealed significant interdecadal
changes in such links. Furthermore, we investigate connection between European precipitation and evaporation
from the surface of the North Atlantic Ocean, the Mediterranean and Baltic seas, and from the European land surface.
We analyze variability of precipitation over Europe on the
basis of data available from the Global Precipitation Climatology Project (GPCP) data set for 1979–2006 [Huffman
et al., 1997; Adler et al., 2003]. In order to get more detailed
information on the summer precipitation variability and to
examine stability of the leading modes of precipitation
during the summer season, we performed analysis for
summer seasonal mean precipitation as well as separate
analyses for each summer month. The paper is organized as
follows. The data used and the analysis methods are
described in section 2. Spatial‐temporal structure of the
leading modes of the summer seasonal and monthly mean
precipitation variability for 1979–2006 and their links to
regional atmospheric circulation are analyzed in section 3.
In section 4, we explore links between regional precipitation
and evaporation during summer season. Finally, summary
and discussion are presented in section 5.
2. Data and Methods
[5] We employed monthly mean global precipitation data
(2.5° × 2.5° latitude‐longitude grid) from the Version‐2 of
the GPCP data set for 1979–2006 [Huffman et al., 1997;
Adler et al., 2003]. The GPCP data set represents a combination of gauge observations and satellite estimates. There
were several reasons to choose this data set. First (and most
important), since the European climate experiences significant interdecadal and longer trend‐like changes, in the
present study, we were interested in characterizing interannual variability during the most recent climate period,
thought to be the warmest since the beginning of instrumental observations [e.g., Trenberth et al., 2007]. Permanently updated GPCP data provide more up‐to‐date
information compared to the Climatic Research Unit (CRU)
data set [New et al., 1999; Mitchell and Jones, 2005] which
has finer spatial resolution but is not so regularly updated.
Moreover, it was shown that for the European region, there
is reasonably good agreement between satellite‐based precipitation products and the CRU data set [e.g., Zveryaev,
2004]. Note the data quality over oceanic/marine regions
in the GPCP data set is somewhat lower (compared to the
land areas) since it is based exclusively on satellite estimates. In the present study the domain of analysis is limited
to latitudes 30°N–75°N and longitudes 15°W–52.5°E.
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[6] In this study we also used evaporation data from the
Woods Hole Oceanographic Institution (WHOI) data set [Yu
and Weller, 2007]. In contrast to other flux products constructed from one single data source, this data set is determined by objectively blending the data sources from
satellite and numerical weather prediction model outputs
while using in situ observations to assign the weights [Yu et
al., 2004; Yu and Weller, 2007]. The WHOI data set provides evaporation data (1° × 1° latitude‐longitude grid) over
the global oceans for 1958–2006. Detailed description of the
data and the synthesis procedure is given by Yu and Weller
[2007] and can be found at the Web site http://oaflux.whoi.
edu. Since observational data over land are rather scarce,
as a complementary data source on evaporation over the
land surface, we used data from the National Centers for
Environmental Prediction (NCEP)/National Center for
Atmospheric Research (NCAR) Reanalysis for 1979–2006
[Kalnay et al., 1996]. These data are diagnostic outputs from
6‐hourly forecasts produced by a numerical weather prediction model in data assimilation mode. Since evaporation
is not directly assimilated, model bias may influence the
reliability of these fields, thereby, limiting the accuracy in
representing links between aspects of the regional water
cycle. It is recognized that the quality of precipitation data in
reanalyses is poor [e.g., Zolina et al., 2004]. Since precipitation influences soil moisture and land surface evaporation, the quality of evaporation in reanalyses is also
questionable. It should be stressed that there is a relaxation
to a seasonal climatology term in the reanalysis surface
water equation [e.g., Roads et al., 1999]. The reason for
this artificial source of water is that preliminary experiments
showed that the reanalysis surface water would have drifted
and would have negatively impacted other near‐surface
and atmospheric variables, in particular, precipitation. Thus,
the reanalysis is being forced toward climatology that is
somewhat inconsistent with its land surface parameterization. We nevertheless hope to obtain reasonable qualitative
assessments of these links within the degree of uncertainty
provided by the reanalysis product.
[7] To assess the links between variability of European
precipitation and regional atmospheric circulation, we use
indices of the major teleconnection patterns that have been
documented and described by Barnston and Livezey [1987].
In our analysis along with links to the NAO, we examine
links to such teleconnections as the East Atlantic pattern,
East Atlantic‐West Russia (EAWR) pattern, and Scandinavian (SCA) pattern, which can also affect European precipitation variability. The data cover the period 1950 to
present. Details on the teleconnection pattern calculation
procedures is given by Barnston and Livezey [1987] and at
the CPC Web site. To reveal the dynamical context of the
leading modes in precipitation variability, we used monthly
sea level pressure (SLP) and 500 hPa heights data from the
NCEP/NCAR Reanalysis for 1979–2006 [Kalnay et al.,
1996].
[8] We examine the spatial‐temporal structure of long‐
term variations in summer monthly and seasonal mean
precipitation over Europe by application of conventional
empirical orthogonal function (EOF) analysis [Wilks, 1995;
von Storch and Navarra, 1995]. To assess links to teleconnections, we used standard correlation analysis. It should
be emphasized that statistical methods used imply that only
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linear relationships between different climate variables (and
mechanisms forming them) in European region are addressed in this study.
3. Leading Modes of the Summer Precipitation
Over Europe and Their Links to Atmospheric
Dynamics
[9] To reveal the leading modes of interannual variability
of precipitation over Europe during summer, we performed
the EOF analysis on time series of the summer (June–July–
August) mean and (separately) June, July, and August
monthly mean precipitation from the GPCP data set for the
period 1979–2006. The time series were linearly detrended,
and anomalies were weighted by the square root of cosine of
latitude [North et al., 1982]. As we earlier mentioned, the
motivation for the separate analyses of the monthly precipitation time series is based on our intention to examine
the stability of the leading EOF modes during the summer
season. We limit our analysis to consideration of the first
two EOF modes because each of the subsequent modes
explains less than 10% of the total precipitation variance and
because significant links between those modes of precipitation variability and regimes of atmospheric circulation
have not been revealed. It should be noted that in August the
leading EOF modes of precipitation are not well separated
according to the North criteria [North et al., 1982]; however,
we include them into our consideration for the sake of
completeness of analysis. Spatial patterns of the first two
EOF modes of precipitation and time series of the
corresponding principal components (hereafter PC) are
shown, respectively, in Figures 1 and 2.
[10] The first EOF mode explains from 17.2% (in June) to
22.8% (in July) of the total variance of precipitation. The
respective spatial patterns (Figures 1a, 1c, 1e, and 1g),
characterized by a tripole‐like structure, depict three action
centers. The major action center extends from the British
Isles to a wide region around the Baltic Sea and further to
eastern Europe and European Russia. Two other centers of
opposite polarity are located to the south (i.e., over Mediterranean region) and north (i.e., over northern Scandinavia)
of the major action center (Figures 1a, 1c, 1e, and 1g).
Structurally, the obtained patterns are very similar to that of
the first EOF mode of the mean summer precipitation from
the CMAP data for 1979–2001 [Zveryaev, 2004]. We note
that the structure of the EOF‐1 patterns demonstrates evident persistence during the summer season. In other words,
structural changes from month to month are not significant,
albeit local (i.e., in action centers) changes in magnitudes of
variability are noticeable. It is worth noting that Casty et al.
[2007] obtained a similar pattern from analysis of a longer
(1766–2000) time series of summer seasonal mean precipitation over Europe. The PC‐1 (Figures 2a, 2c, 2e, and 2g),
displaying temporal behavior of this mode, demonstrates
evident correspondence with the NAO index in all considered months and in analysis of seasonal mean precipitation.
Moreover, high (and statistically significant according to the
Student’s t test [Bendat and Piersol, 1966]) correlations
between respective PCs and the NAO index (Table 1)
clearly indicate that during the entire summer season EOF‐1
of European precipitation is associated with the NAO. It
should be noted, however, that summer NAO is essentially
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different (in terms of its spatial structure) from its winter
counterpart [Barnston and Livezey, 1987]. In particular,
location of the summer NAO action centers is quite different
[Wanner et al., 1997; Mächel et al., 1998; Portis et al.,
2001]. Hence, the NAO‐associated summer precipitation
patterns (Figures 1a, 1c, 1e, and 1g) are also principally
different from the winter dipole‐like patterns [e.g., Hurrell,
1995; Zveryaev, 2004].
[11] The second EOF mode of summer precipitation over
Europe accounts for 12.4%–15.3% of its total variance. The
spatial pattern of this mode (Figures 1b, 1d, 1f, and 1h) in
general represents a meridional dipole characterized by the
coherent precipitation variations over the northern part of
European Russia and Scandinavia and opposite variations
over the remaining part of Europe. In particular, the pattern
is well depicted in July (Figure 1f). However, in contrast
to the first EOF, there are evident month‐to‐month changes
in the structure of the second EOF mode. For example, in
June (Figure 1d), the largest loadings are observed over
western Europe and western Scandinavia, whereas in July
(Figure 1f), they are revealed over eastern Europe and
European Russia. In August (Figure 1h), the entire dipole
demonstrates zonal rather than meridional orientation.
Therefore, the second EOF mode of precipitation is less
stable during the summer season compared to the first
mode. Figures 2d, 2f, and 2h and results of correlation
analysis (Table 1) imply that this mode of European precipitation is driven mainly by the SCA teleconnection pattern [Barnston and Livezey, 1987], consisting of the major
action center over Scandinavia, and minor action centers of
opposite polarity over western Europe and eastern Russia.
Note, however, the second EOF mode of summer mean
precipitation does not demonstrate a significant link to the
mean summer SCA index. A possible reason for this is
that the mean summer SCA index is defined not as a
respective EOF mode obtained from analysis of summer
mean 500 hPa geopotential heights (CPC does not provide
such seasonal indices) but as the average from the SCA
indices estimated for June, July, and August. Since interannual behavior of these monthly indices is rather different
(Figures 2d, 2f, 2h), their average can hardly be viewed as
a representative parameter reflecting interannual variability
of summer mean atmospheric circulation.
[12] We further briefly analyze the leading EOF modes of
the SLP and 500 hPa fields in Atlantic‐European sector and
their links to European precipitation. Since there is general
consistency between leading EOF modes of precipitation
(and other considered climate variables) estimated for different summer months (and for the seasonal mean), and in
order to avoid repetition, we show relevant figures only for
July (central summer month). It should be stressed, however, that further analysis in this and next section was performed for each summer month.
[13] The spatial patterns of the EOF‐1 of July 500 hPa
heights and SLP (Figures 3a and 3c) represent the summer
NAO and show a good agreement with the July NAO pattern presented by Barnston and Livezey [1987]. The major
action center covers a large part of Europe (Figures 3a and
3c), and along with the respective pattern of July precipitation (Figure 1e), suggests that an anticyclonic (cyclonic)
anomaly results in deficient (excessive) precipitation over a
large portion of Europe. The PCs of this mode (not shown)
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Figure 1. Spatial patterns (mm/d) of the first two EOF modes of the (a and b) summer mean, (c and d)
June, (e and f) July, and (g and h) August GPCP precipitation (1979–2006). Red (blue) color indicates
positive (negative) values.
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Figure 2. Principal components of the first two EOF modes of the (a and b) summer mean, (c and d)
June, (e and f) July, and (g and h) August GPCP precipitation (1979–2006). Blue (green) curves depict
the NAO (SCA) index.
are strongly correlated to the July NAO index (0.73 and 0.49
for SLP and 500 hPa, respectively) and to PCs of the EOF‐1
of July precipitation (0.85 and 0.91 for SLP and 500 hPa).
[14] In July, the spatial patterns of the EOF‐2 of 500 hPa
and SLP (Figures 3b and 3d) are characterized by two
dominating action centers located over the northeastern
North Atlantic and over European Russia. Minor action
centers of opposite polarity over Scandinavia, Greenland,
and western North Atlantic are seen in the EOF‐2 pattern for
SLP (Figure 3d). Structurally, the obtained EOF‐2 patterns
are similar to the EAWR pattern obtained by Barnston and
Livezey [1987] and referred to as the Eurasia‐2 pattern in
their study. Respective PCs are significantly correlated to
the July EAWR index (0.74 and 0.72 for SLP and 500 hPa,
respectively) but not correlated to PCs of the second EOF
mode of July precipitation because latter, as shown above, is
associated with the Scandinavian teleconnection.
[15] Summarizing results of this section, we note that
during summer the first EOF mode of European precipitation is stable (in terms of its month‐to‐month variations) and
Table 1. Correlation Coefficients Between PC‐1 and PC‐2 of
Summer, June, July, and August Precipitation and Indices of
Teleconnection Patternsa
Summer
NAO
SCA
June
July
August
PC‐1
PC‐2
PC‐1
PC‐2
PC‐1
PC‐2
PC‐1
PC‐2
0.67
0.10
0.14
0.30
0.68
−0.16
0.48
0.76
0.50
−0.21
−0.12
0.65
0.63
−0.26
0.04
0.49
a
Coefficients, shown in bold, are statistically significant at the 95%
significance level.
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Figure 3. Spatial patterns of the first two EOF modes of July (a and b, in meters) 500 hPa and (c and d,
in millibars) SLP fields (1979–2006). Red (blue) color indicates positive (negative) values.
is strongly linked to the major regional climate signal: the
NAO. The second EOF mode of regional precipitation is
less stable and demonstrates some structural changes during
the summer season. Our results suggest that the major driver
for this mode is the SCA teleconnection pattern [Barnston
and Livezey, 1987], which is not among the leading
modes of the regional atmospheric circulation during summer season.
4. Links Between European Precipitation
and Regional Evaporation
[16] In this section we examine links between European
precipitation and evaporation in four regions that can
potentially impact variability of European precipitation
during the warm season. These regions are the North
Atlantic Ocean, the Baltic and Mediterranean seas, and
Europe (i.e., European land surface). We first reveal the
leading modes of evaporation in each region by applying
EOF analysis to detrended time series of evaporation from
the WHOI data set (for oceanic/marine regions) and from
the NCEP/NCAR reanalysis (for European land surface) for
1979–2006. Spatial patterns of the first and second EOF
modes of evaporation for the Baltic Sea, Mediterranean Sea,
and Europe are shown, respectively, in Figures 5–7. Note,
the spatial patterns obtained for other summer months are
similar to those presented in Figures 5–7. Further, we analyze links between leading EOF modes of evaporation in
aforementioned regions and leading modes of precipitation
over Europe. Since we did not find statistically significant
links between large‐scale European precipitation variability
and evaporation in the North Atlantic during summer, we
exclude this region from our further analysis. Note, however, that local precipitation variability in some European
regions (e.g., northern Scandinavia) can be influenced by
the North Atlantic moisture transport.
[17] Since our analysis of the leading modes of precipitation and evaporation (and their relationships) characterize
variations of some fractions of total precipitation (or evaporation), it is of interest first to look and compare lump
precipitation/evaporation in the regions of interest and their
interannual variations. For July, the mean total water flux
(and its standard deviation, both in km3/d) is 19.3 (2.85) for
European precipitation, 31.2 (1.60) for European evaporation, 8.16 (1.02) for Mediterranean evaporation, and 1.51
(0.31) for the Baltic Sea evaporation. Thus, it is evident that
the major players for the regional hydrological cycle are the
European land area and the Mediterranean Sea. Figure 4
depicts anomalies of the total water flux estimated for
European precipitation and evaporation and for evaporation
from the Mediterranean/Black Seas and Baltic/North Seas.
Correspondence between the presented time series is obvious. Correlation between European precipitation and evaporation is 0.53. When Mediterranean evaporation is added to
European evaporation, correlation with precipitation increases to 0.58. Adding of Baltic/North Sea evaporation
does not affect significantly correlation with European
precipitation (0.57). This suggests that Mediterranean
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Figure 4. Total water flux anomalies (in km3/d) for July estimated for different regions.
evaporation may explain a significant portion of European
precipitation variance; however, the role of local (i.e., from
European land surface) evaporation is likely to be most
important. We note that these (rather rough) estimates just
provide useful background for our further analysis, whereas
accurate balance estimates for regional hydrological cycle
are beyond the scope of the present study.
[18] We extended slightly the domain of analysis for the
Baltic Sea region since both the North Sea and Baltic Sea
are influenced by the same atmospheric circulation patterns
(Figure 3) and because the amount of grid points covering
the Baltic Sea is relatively low. In July, the first EOF mode
of evaporation in the extended Baltic/North Sea region explains about half (51.9%) of its total variability. Its spatial
pattern reflects coherent variations of evaporation over the
entire domain of analysis (Figure 5a). Although there is
significant correlation to PC‐1 of precipitation in August, in
general, principal components (not shown) of this mode do
not demonstrate significant correlations to PC‐1 and PC‐2
of precipitation (Table 2), suggesting that this mode does
not affect significantly large‐scale variability of European
precipitation during summer. The second EOF mode of
evaporation in the Baltic/North Sea region accounts for
18.7% of its total variability in July. Its spatial pattern depicts a dipole with opposite variations of evaporation in the
Baltic Sea and the North Sea (Figure 5b). Such a pattern
presumably reflects more local (compared to the first EOF
mode) forcings of the regional evaporation variability.
Principal components of this mode (not shown) demonstrate
significant correlation to the EOF‐1 of European precipitation in June and July, and to the EOF‐2 in August (Table 2),
suggesting an influence of this mode on variability of
regional precipitation. However, since the EOF‐2 explains a
relatively low fraction of the total evaporation, we presume
that this influence is not large.
[19] The first EOF mode of evaporation from the surface
of the Mediterranean Sea in July explains 45.6% of its total
variability. The spatial pattern of this mode is characterized
by coherent variations of evaporation over the entire Mediterranean Sea (Figure 6a). Principal components (not
shown) of this mode correlate significantly to PC‐1 of
precipitation over Europe (Table 2), suggesting an essential
influence of this mode on summertime variability of
regional precipitation. More specifically, Figures 1e and 6a
indicate that below (above) normal precipitation over a large
part of Europe is associated with decreased (increased)
evaporation from the surface of the Mediterranean Sea.
Dynamical background for this association (Figure 3c)
suggests that the positive (negative) phase of the summer
NAO leads to reduced (enhanced) advection of the Mediterranean moisture into eastern Europe and European Russia, resulting in below (above) normal precipitation in these
regions. Note, however, that in June and August the first
EOF mode of Mediterranean evaporation is associated with
the second EOF of European precipitation (Table 2). The
EOF‐2 accounts for 21.3% of total variability of evaporation
in the Mediterranean Sea in July. Its spatial pattern is
characterized by the zonal dipole with opposite variations of
evaporation in the western and eastern parts of the sea
(Figure 6b). Principal components of the EOF‐2 (not
shown) demonstrate significant correlation to the EOF‐1 of
European precipitation in July and August (Table 2). Thus,
our results suggest that both the first and the second EOF
modes, explaining together about 67% of total variability of
Mediterranean evaporation, affect summertime variability of
precipitation over Europe. Although aforementioned correlations are almost equal, the influence of the first EOF mode
is indeed significantly larger since it explains double the
fraction of the total variability of evaporation.
[20] The spatial pattern of the EOF‐1 of evaporation from
the European land surface is characterized by the major
action center covering almost all of Europe from the Iberian
Peninsula and France to Scandinavia and European Russia
where the largest loadings are revealed (Figure 7a). A
minor action center of opposite polarity is revealed over the
Balkans and eastern Mediterranean/Black Sea region. This
mode explains 24.9% of the total variability of regional
evaporation. Principal components of this mode show high
correlations to the PC‐1 of European precipitation in June,
July, and August (Table 2) implying coupling of the leading
modes of European precipitation and evaporation during the
warm season. Above‐detected high correlations (the largest
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Figure 5. Spatial patterns (mm/d) of the first two EOF modes of July evaporation in the Baltic
Sea‐North Sea region (1979–2006). Red (blue) color indicates positive (negative) values.
among those considered in our analysis, see Table 2),
however, does not point to causal relationships between
regional precipitation and land surface evaporation and may
indicate a positive feedback when enhanced precipitation
results in increased soil moisture and evaporation, which
amplifies regional precipitation. In this regard, it is of
interest to compare amounts of precipitation and evaporation
and magnitudes of their interannual variability. Over central/
eastern Europe and European Russia (i.e., regions of the
largest variability of the summer precipitation, see Figure 1e)
July precipitation values (not shown) vary from 2.5 mm/d to
3.5 mm/d, whereas reanalysis evaporation in this region
varies in the range 3.5–4.5 mm/d, thus, exceeding regional
precipitation. On the other hand, standard deviations (not
shown) of precipitation (1.0–1.4 mm/d) in the region are
approximately twice those of evaporation (0.4–0.7 mm/d).
Values of evaporation and its standard deviations in the
Mediterranean Sea are comparable to those over land. The
largest July evaporation (reaching 3.6 mm/d) is observed in
the eastern Mediterranean Sea. Overall, this suggest, that
both precipitation and local evaporation may affect each
other. Although the magnitudes of interannual variability of
evaporation are smaller than those of precipitation, they are
evidently nonnegligible (see also Figure 4). The second EOF
mode of evaporation from the European land surface in July
explains only 12.8% of its total variability. Its spatial pattern
represents a meridional dipole with opposite variations of
evaporation north/south off approximately 53°N–55°N latitude (Figure 7b). Only in August, principal components of
this mode significantly correlated to the second EOF mode
of regional precipitation (Table 2).
Table 2. Correlation Coefficients Between PC‐1 and PC‐2 of
June, July, and August Precipitation and Evaporation in Different
Regionsa
North Atlantic
Baltic
Mediterranean
Europe
Europe
Precipitation EVA1 EVA2 EVA1 EVA2 EVA1 EVA2 EVA1 EVA2
PRE1
PRE2
PRE1
PRE2
PRE1
PRE2
(Jun)
(Jun)
(Jul)
(Jul)
(Aug)
(Aug)
−0.03
0.33
−0.14
−0.10
−0.35
0.20
−0.10
0.02
0.04
−0.16
0.43
0.26
0.47 0.31
0.01 0.64 0.05
0.12 0.51
0.21 0.13 0.29
0.48 0.43 −0.42 0.78 0.34
0.17 −0.21 0.23 −0.26 0.30
−0.28 −0.03 −0.44 0.62 0.20
0.50 0.49
0.22 0.10 0.59
a
Coefficients, shown in bold, are statistically significant at the 95%
significance level.
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Figure 6. Spatial patterns (mm/d) of the first two EOF modes of July evaporation from the surface of
the Mediterranean Sea (1979–2006). Red (blue) color indicates positive (negative) values.
[21] To summarize results of this section, we note that our
analysis suggests that, in contrast to the winter season,
during summer, the evaporation in the North Atlantic does
not affect continental‐scale interannual variability of precipitation over Europe. However, smaller‐scale variability
of precipitation, particularly in some coastal regions, can be
significantly affected by this factor [e.g., Lenderink et al.,
2009]. Our analysis indicates a significant role of land
surface evaporation in the variability of European precipitation during the warm season. This result supports recent
findings based on model simulations [Koster and Suarez,
1995; Schär et al., 1999; Seneviratne et al., 2006]. Note,
however, that in contrast to the North Atlantic, Baltic, and
Mediterranean seas where observation‐based data were
used, for the land surface, we used evaporation from
reanalysis products with well known limitations. We also
found statistically significant links between evaporation in
the Baltic and Mediterranean seas and interannual variability
of precipitation over Europe. However, we believe that the
major regions affecting (through evaporation) regional precipitation during the warm season are the European land
area and the Mediterranean Sea, while evaporation in the
Baltic Sea plays a minor role. Overall, results of this section
suggest that in contrast to the winter season when moisture
advection from the North Atlantic into the European region
plays a dominant role in regional precipitation variability,
during boreal summer, local processes make significant
contribution to the interannual variability of European
precipitation.
5. Summary and Discussion
[22] In the present study we analyzed the leading modes
of interannual variability of summertime precipitation over
Europe based on the data from the GPCP data set for 1979–
2006 [Huffman et al., 1997; Adler et al., 2003]. We also
investigated the relation of these modes to regional atmospheric circulation, and their links to evaporation in the
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Figure 7. Spatial patterns (mm/d) of the first two EOF modes of July evaporation from the European
land surface (1979–2006). Red (blue) color indicates positive (negative) values.
North Atlantic Ocean and Baltic and Mediterranean seas, as
well as to evaporation from the European land surface.
[23] It is shown that the first EOF mode of European
precipitation is rather stable (in terms of its spatial‐temporal
structure) during the summer season and is characterized by
a tripole‐like pattern with large coherent variations over a
wide region extending from the British Isles to European
Russia. Relatively weak precipitation variations of opposite
sign are revealed north and south of the above region. This
mode is associated with the summer NAO [e.g., Zveryaev,
2004; Folland et al., 2009]. Since anomalies in atmospheric circulation during summer are not as large as during
winter and because precipitation is one of the most variable
climate parameters, it is not obvious to expect the revealed
stability of the first mode of summer precipitation. For the
first time, we show that during recent decades, the second
EOF mode of summer precipitation (characterized by
meridional dipole structure) is less stable and is linked to the
Scandinavian teleconnection [Barnston and Livezey, 1987].
Note that analysis performed for the century‐long time
series of precipitation [Zveryaev, 2006] did not reveal such a
link. Moreover, it was shown that different mechanisms can
be major drivers for European precipitation variability during
different climate periods [Zveryaev, 2006; 2009]. In particular, it was demonstrated that during periods of weak NAO
influence on European precipitation, the Scandinavian teleconnection played a role of major driver for regional precipitation variability in spring and fall [Zveryaev, 2009].
Therefore, our findings characterize the most recent climate
period which is recognized as the warmest period in the
history of instrumental observations. Also, it should be
emphasized that the first two EOF modes considered in the
present study describe together up to 35% of total variability
of European precipitation. Thus, a substantial portion of
summertime precipitation variability over Europe remains
undescribed, and mechanisms that drive this part of precipitation variability are not clear, implying necessity of
further studies in this direction. It is clear that present study
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based on the analysis of monthly data has certain limitations
in investigation of such mechanisms. In this regard, an
analysis of summertime precipitation variability at shorter
(e.g., synoptic, subsynoptic, etc.) time scales based on data
having higher temporal resolution looks very promising and
can potentially shed more light on the mechanisms driving
regional precipitation variability.
[24] Analysis of links between European precipitation and
evaporation has shown that, in contrast to the winter season,
when regional precipitation variability is mostly determined
by the NAO‐driven moisture advection from the North
Atlantic, summertime continental scale variability of precipitation is not associated with evaporation in the North
Atlantic. On the contrary, our results suggest a significant
role of the local processes, in particular, land surface
evaporation, in variability of regional precipitation during
the warm season, supporting recent model‐based results
[e.g., Schär et al., 1999; Seneviratne et al., 2006]. Because
we used in our study reanalysis data having well‐known
limitations, further analysis of the role of land surface
evaporation in interannual variability of European precipitation during the warm season is needed. In particular, an
analysis (based on higher temporal and spatial resolution
data) of the relative roles of the local evaporation and
regimes of regional atmospheric circulation focused on different time scales would be of great interest since these roles
can vary significantly depending on time scales. It should be
noted that a revealed links between the leading modes of
regional precipitation and land surface evaporation does
not indicate causal relationships between these variables and
may reflect a positive feedback when enhanced precipitation
leads to an increase of soil moisture and evaporation, which in
turn amplifies regional precipitation. Thus, to get deeper
insight into causal relationships between European precipitation and land surface evaporation, model experiments are
highly desirable. For example, simulations of European climate with high‐resolution regional climate models [e.g.,
Vidale et al., 2003, 2007] look very promising. Although
there is considerable spread in the models’ ability to represent
the observed summer climate variability, we believe that
further experiments, for instance, applying climatological
ancillary fields to restrict the variability of surface moisture
fluxes and analyzing the dependence of model precipitation
on such forcings, could provide informative results and
make causal relationships in the regional hydrological cycle
clearer.
[25] We also found significant links between summertime
European precipitation and evaporation in the Mediterranean Sea which also (along with the land surface) can be
viewed as a local (rather than remote) source of moisture. It
seems that the influence of the Baltic Sea evaporation on
regional precipitation is not large (although statistically
significant links are detected) and probably limited to the
Baltic region.
[26] The present study highlights mechanisms driving
summertime interannual variability of precipitation over
Europe. Since the summertime NAO is structurally different
from that for other seasons, its impact on summer precipitation variability over Europe is also principally different.
We found that during summer the leading modes of regional
precipitation are not associated with evaporation in the
North Atlantic but linked to local processes such as evap-
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oration from the European land surface and from the surface
of the Mediterranean Sea. However, since our assessment of
the links to land surface evaporation is limited to reanalysis
products, we hope that further diagnostic studies of the
observational data as well as model experiments will allow
obtaining more accurate estimates of these links.
[27] Acknowledgments. This research was supported by the Royal
Society grant (International Incoming Short Visits, IV0866722). A major
part of the present study has been performed during IIZ work at the Environmental Systems Science Centre, University of Reading, as a visiting scientist.
IIZ was also supported by the Russian Ministry of Education and Science
under the Federal Focal Program “World Ocean” (contract 01.420.1.2.
0001) and Russian Academy of Sciences Research Program “Fundamental
problems of Oceanology.” Discussions with Sergey Gulev are appreciated.
The NCEP data was extracted from NOAA‐CIRES Climate Diagnostics
Center. The manuscript was significantly improved by the constructive
comments of three anonymous reviewers.
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R. P. Allan, Department of Meteorology, University of Reading,
Reading, Berkshire, RG6 6BB, UK.
I. I. Zveryaev, P.P. Shirshov Institute of Oceanology, RAS, 36,
Nakhimovsky Ave., Moscow, 117997, Russia. ([email protected])
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